1. Field of the Invention
The present invention generally relates to a method of and an apparatus for controlling a movement of a sliding piston of a fluid-operated cylinder to be used with hydraulic machines, and more particularly, relates to a method of and apparatus for controlling a molding process by controlling a position and an output force of a sliding-piston of a hydraulic cylinder used as an injection cylinder for actuating an injection plunger and a pressurizing cylinder for operating a melt pressing plunger of die casting and injection molding machines and an extrusion cylinder of an extrusion machine.
2. Description of the Related Art
Fluid-operated cylinders with a sliding-piston are conventionally used as an injection cylinder for injecting molten metal or plastic into dies of a die casting machine or molds of a plastic molding machine. The fluid-operated cylinders are also used as a pressurizing cylinder for pressurizing the molten metal or plastics injected in the dies or molds to produce a melt pressing effect to thereby compensate for shrinkage of the molten metal or plastics in the die cavity. This pressure imposed by the pressurizing cylinder will be hereinafter referred to as "a squeeze pressure". The movement of the sliding-piston of the fluid-operated cylinder is controlled in such a manner that a change in the position of the sliding-piston with respect to a reference position thereof (i.e., the stroke of the sliding-piston) is detected by a position detector, and a detection signal output by the position detector is fed back, via a programmed speed setter, to a servo valve for controlling moving speed of the sliding-piston of the fluid-operated cylinder to a predetermined value at a predetermined position thereof. It has been found, however, that the control of the above-mentioned moving speed of the sliding-piston of the fluid-operated cylinder with respect to the stroke of the piston is not adequate for controlling the casting or molding process of the die casting or injection mold machines from the view point of the need for precise and high quality castings and molded products.
In this connection, U.S. Pat. No. 4,840,557 to Kenji Ishimoto et al discloses a vertical die casting machine adopted for producing disc wheel castings for cars.
FIGS. 21A through 21E illustrate a casting process of the disc wheel casting by the vertical die casting machine of U.S. Pat. No. 4,840,557. This die vertical casting machine has vertically movable upper and lower dies 141 and 142, and plurality of laterally movable cores 143 which are mated together and clamped by a pressing mechanism via upper and lower platens, to define a die cavity 144. As illustrated in FIG. 21A, when the die cavity 144 is defined between the dies 141 and 142, a melt 146 such as melted aluminum and magnesium is injected from inside a sleeve 147 into the cavity 144 by an upward movement of an injecting plunger 148 having an injection tip 148a at an upper end thereof, which is in turn operated by an injection cylinder (not illustrated in FIGS. 21A through 21E). When a predetermined time has lapsed (i.e., delaying start of pressurizing process for a predetermined time) after completion of the injection of the melt 146 into the die cavity 144, a melt pressing plunger 145 (it will be hereinafter referred to as a squeezing plunger throughout the description of the present application.) is moved down into the die cavity 144 to pressurize the melt 146 therein and feed the die cavity 144, as shown in FIGS. 21B and 21C, by the actuation of a pressurizing cylinder (not shown). Namely, before the melt 146 solidifies, the squeezing plunger 145 is lowered from an intermediate position illustrated in FIG. 21B to a lower position illustrated in FIG. 21C at which the lower end portion of the squeezing plunger 145 seals a gate 149 of the lower die 142 while leaving a thin film of solidified melt 146a around the gate 149. Subsequently, when the injected melt 146 is completely solidified by cooling, the squeezing plunger 145 is moved slightly upward from the gate sealing position of FIG. 21C, and the upper and lower dies 141 and 142 are separated and opened as illustrated in FIG. 21D. Accordingly the solidified melt 146 is vertically split into two parts at the position of the gate 149 where the thin film of solidified melt is formed. Thereafter, as illustrated in FIG. 21E, the cores 143 are retracted from the dies 141 and 142, and a disc wheel casting 100 is ejected from the upper die 141, i.e., an ejector die, by ejector pins 99.
In the above-mentioned die casting process according to the prior art vertical die casting machine, a squeeze pressure exerted by the squeezing plunger 145 is controlled for a time as illustrated in the graph of FIG. 22, in which the abscissa indicates time and the ordinate indicates a change in the squeeze pressure Psq. An injection pressure Pm denotes a pressure level at which the melt 146 is injected into the die cavity 144. From FIG. 22, it is understood that a pressure of the squeezing plunger 145, i.e., the squeeze pressure Psq, is applied step-wise in four different pressure stages. The time 0 indicates a starting time of the application of the squeeze pressure Psq and corresponds to a time after the lapse of fractions of one second or approximately 1.0 second (a predetermined lag time) from the time of completing the injection of the melt 146 into the die cavity 144. This starting time is determined by the operation of an appropriate timer device started by detecting the time of completion of the injection of the melt 146, using a detector such as a pressure detector for detecting a pressure within the die cavity 144 or a rise in the operating pressure of the injecting cylinder, and a measuring unit for measuring an amount of stroke of the injecting plunger 148.
Further, an interval between the time 0 and a time t.sub.1 in FIG. 22 indicates a region in which the squeezing plunger 145 is moved down while breaking a chill surface layer of the melt 146 formed when the injected melt 146 comes into contact with inner surfaces of the upper and lower dies 141 and 142. During this interval, the squeezing plunger 145 moved downward in the die cavity 144 while squeezing the solidifying melt 146, and therefore, a very high squeeze pressure must be exerted by the squeezing plunger 145 actuated by the squeezing cylinder. An interval between the times t.sub.1 and t.sub.2 indicates a region in which the melt 146 is gradually cooled to be solidified from the melted state. As a result, in compliance with the cooling of the melt 146, the melt per se shrinks, and therefore, the squeezing plunger 145 is moved down in the melt 146 during the interval between t.sub.1 and t.sub.2 to exert a relatively low squeeze pressure and thereby compensate for the shrinkage of the melt 146. Namely, the squeeze pressure Psq during the interval between the time t.sub. 1 and the time t.sub.2 is set to be either slightly larger or less than the injection pressure Pm, depending on the shape of the casting 100. In some cases, the squeeze pressure during this interval is set to be approximately equal to the injection pressure Pm. An interval between the time t.sub.2 and t.sub.3 of FIG. 22 indicates a region in which the melt 146 in the die cavity 144 is cooled and partly solidified to cause shrinking of the cooled melt 146. This shrinkage of the melt permits holes or cavities to be generated inside the melt, and therefore, at the time t.sub.2, a higher squeeze pressure is applied by the squeezing plunger 145 to prevent the generation of the holes or cavities inside the solidified melt 146. An interval between the times t.sub.3 and t.sub.4 indicates an additional or final region in which the squeezing plunger 145 is moved down to the position of the gate 149 of the lower die 142, to permit the formation of the above-mentioned thin film of the solidified melt 146a.
When the casting process is started when the upper and lower casting dies 141 and 142 are cold, the squeezing plunger 145 and the upper die 141 are heated by the melt 146 injected in the die cavity 144, and thermally expanded, respectively. Nevertheless, the amount of thermal expansion of the squeezing plunger 145 and the upper dies 141 is different, because of differences in the coefficients of the thermal expansion thereof, the cooling method applied, and an amount of heat absorbed by both elements. As a result, when the squeezing plunger 145 is repeatedly moved down into the die cavity 44 during a number of casting cycles, a frictional resistance at a portion "A" of FIG. 21B, to which the plunger 145 is subjected, changes, and further, intrusion of the melt 146 into a clearance between the plunger 145 and a bore of the upper die casting 141 also causes a change in a resistance to the downward movement of the squeezing plunger 145 at a portion designated by "B" in FIG. 21B, in respective squeezing operations of the squeezing plunger 145.
Therefore, when the operation of the squeezing plunger 145 is controlled in the conventional method to exert the squeeze pressures Psq in a step-wise manner as shown FIG. 23, an actual movement of the plunger 145 into the die cavity, i.e., the stroke of the squeezing plunger 145, must be gradually changed in response to a change in the above-mentioned resistance acting against the squeezing plunger 145 during a number of casting cycles of the die casting machine of FIGS. 21A through 21E.
FIG. 23 illustrates that, when the casting operation of the die casting machine is started when the upper and lower dies 141 and 142 are cold by employing the conventional method of controlling squeeze pressures Psq in the step-wise manner shown in FIG. 22, the stroke St of the squeezing plunger 145 is gradually shortened in response to an increase in the number of casting cycles, as shown by an arrow. Namely, in the first casting operation of the die casting machine, the stroke "St" of the squeezing plunger 145 with respect to the elapse of the time "t" changes according to a curve (a) of FIG. 23. Nevertheless, when the die casting machine is continuously operated to perform a number of casting operations, the stroke of the squeezing plunger 145 operated by the non-illustrated pressurizing cylinder becomes short, as shown by the curves (b), (c) and (d) of FIG. 23, in response to an increase in the number of casting cycles. This occurs due to the afore-mentioned increase in the resistance against or a load applied to the squeezing plunger 145.
Since the application of the squeeze pressure by the squeezing plunger 145 is effected to supply a feed needed to compensate for a void or cavity in the melt 146 in the die cavity 144, due to the shrinkage of the melt 146, the above-mentioned shortening of the stroke of the squeezing plunger 145 in response to an increase in the number of the casting operations fails to compensate for the void of the melt 146 in the die cavity 144, and as a result, high quality castings cannot be obtained.
In addition, when the stroke of the squeezing plunger 145 is shortened, and therefore the plunger 145 cannot be moved to the position at which it seals the gate 149 as shown in FIG. 21C, the gate 149 will be filled with the solidified melt 146, i.e., the thin film-like cylindrical portion 146a of the solidified melt normally formed at the gate 149 of the lower casting die 142 cannot be obtained. Accordingly, as can be seen in FIG. 21D, when the upper and lower casting dies 141 and 142 are opened vertically to take the casing 100 from the die cavity 144 without removing the cores 143, a large force is needed to separate the solidified melt at the gate 149 of the lower casting die 142, and since the cores 143 are not yet taken out, the solidified melt 146 is pulled downward by a large force exerted by the downward moving lower casting die 142 to separate the solidified melt 146 at the gate 149. Therefore, the casting 100 might be deformed as shown by two dots chain lines in FIG. 21D.
From the foregoing description of the die casting process according to the prior art, it can be understood that, when the step-wise method of control of the squeeze pressure is adopted, it is necessary for an operator to monitor the operation of the squeezing plunger 145 and manually adjust a force applied to the squeezing plunge 145, i.e., a pressing force exerted by the pressurizing cylinder, and the timing for changing the squeeze pressure levels. This is very cumbersome and difficult and a satisfactory adjustment of the squeeze pressure levels by the operator can not be practically effected. Therefore, there is an urgent need for an improvement of the prior art method of controlling the operation of the die casting and molding machines, as well as the operation of a fluid-operated cylinder with a sliding piston to be used as an actuator of the die casting or plastic molding machine.